The invention relates to a process for producing a composite structure in which a metal foam core is surrounded by a metal body. It also relates to a component produced using the process.
In accordance with the prior art, DE 195 26 057 has disclosed a process for producing a composite structure. In this process, the surface of a metal foam core is compacted with heating, so that only fine cracks and holes remain in the surface. Then, a thermal spraying process is used to apply a metal layer to the surface. In this process, the contour of the component is predetermined by the contour of the metal foam core. Complex contours cannot be produced or can only be produced with great difficulty.
DE 196 50 613 has disclosed a component with a metal foam core and a process for its production. The metal foam core is surrounded by a metal foil and then a casting material is cast around it. The metal foil has the purpose of preventing the molten material from penetrating into the pores of the metal foam core. The known process is complex, since the metal foam core has to be surrounded with the metal foil in such a manner that a seal is formed. This requires manual action.
It is an object of the invention to eliminate the drawbacks of the prior art. In particular, it is intended to describe a process for producing a composite structure which can be carried out as easily and inexpensively as possible.
A further aim of the invention is provide an automatable process for producing a composite structure.
This object is achieved by the features of claim 1. Expedient configurations will emerge from the features of claims 2-10.
The invention provides a process for producing a composite structure, in which a metal foam core (1) is surrounded by a metal body, comprising the following steps:
a) production of a metal foam core with a substantially continuous surface,
b) insertion of the metal foam core into a die-casting die,
c) filling of the die-casting die under a first casting pressure,
d) reduction of the first casting pressure before the die-casting die has been filled,
e) complete filling of the die-casting die, during which process the first casting pressure is reduce to zero or virtually zero, and
f) application of a second casting pressure and holding of his pressure for a predetermined holding time.
The proposed process is simple and inexpensive to carry out. It is not absolutely imperative to provide a metal foil or the like to seal the surface pores of the metal foam. According to the invention, this is achieved in particular by the first pressure being reduced or lowered to zero or virtually zero, for example by reducing the piston advance velocity of the die-casting device, before the die-casting die has been completely filled. A pressure peak which occurs in the continuous die-casting process, in particular at the time of complete filing of the die-casting die, is avoided. It is assumed that this measure leads to the formation of a solidification layer on the surface of the metal foam core, which surprisingly, despite the subsequent application of a second casting pressure, prevents molten material from penetrating into the metal foam core.
The metal foam core is produced using known processes. For this purpose, by way of example, an alloy which has been mixed with a metal hydride, preferably a titanium hydride, and is in the form of sheet-metal strips, pieces or granules, is introduced into a closed die. The die is heated and in the process the alloy melts. The metal hydride releases gas and in the process produces foaming. A metal foam core which has been produced in this manner has a surface or skin which is substantially continuous, i.e. contains fine pores and cracks.
The term casting pressure is understood as meaning the pressure which prevails in the shot sleeve. The casting pressure generally differs from the pressure in the die-casting die acting on the metal foam core which is accommodated therein. This difference is brought about, for example, by the geometry of the die-casting die, e.g. its gate, or by dynamic effects, such as friction forces. The casting pressure is usually greater than the pressure which is thereby exerted on the metal foam core.
The first and/or second casting pressure is expediently applied in accordance with a predetermined pressure/time curve. This defines, for example, the rate at which the pressure increases, the second casting pressure and the holding time of this pressure.
According to an advantageous configuration, the first casting pressure is lower than the second casting pressure. The first casting pressure is expediently reduced as soon as the die-casting die is at least 90% full by volume. The pressure which is produced on the metal foam core by the second casting pressure is advantageously lower than its compressive strength. The pressure which is generated on the metal foam core by the first casting pressure is expediently less than 25 bar, and the pressure which is produced on the metal foam core by the second casting pressure is greater than 25 bar. The second casting pressure is preferably between 200 and 700 bar.
The first casting pressure is used to substantially completely fill the die-casting die. During this phase, the molten material is able to fill the main open volume of the die-casting die. In the process, no significant pressure is exerted on the metal foam core. The second casting pressure is only applied when the die casting die has been completely filled. The pressure produced by the second casting pressure acts on the metal body and the metal foam core. It is lower than the compressive strength of the metal foam core, in order not to destroy the structure of this core, and high enough to close up pores which have remained in the metal body.
According to a further configuration, spacers are formed integrally on the metal foam core. They are expediently designed as web-like elevations. This further simplifies the proposed process, makes it less expensive and creates additional design options for the component geometry.
According to a further design feature, there is provision for the metal foam core to be provided with a heat-resistant coating before step b). This coating can be produced by thermal spraying or by dipping into a ceramic slip. The thermal spraying may take place, for example, by means of flame spraying, e.g. aluminum wire flame spraying. As an alternative to wire flame spraying, it is also possible to use other high-speed flame spraying processes, for example vacuum plasma spraying. By way of example, the slip used may be a MgAl spinel slip. The slip adheres well to the surface of the metal foam core. The abovementioned features additionally prevent molten material from entering the pores at the surface of the metal foam core during the die-casting operation.
According to a further particularly advantageous configuration, a vacuum is applied to the die cavity surrounded by the die-casting die after step b). It is advantageous for the die cavity to be as evacuated as far as possible. Good results are achieved when a vacuum in the range from 5 to 50 mbar, preferably from 10 to 30 mbar, is applied to the die cavity. The vacuum is expediently applied to the die cavity until the die-casting die has been completely filled with molten material. The application of the vacuum can be disconnected by the molten material, when the die-casting die is completely full, penetrating into associated runners, where it closes a vacuum relief valve arranged there. The application of vacuum to the die-casting die allows simple and rapid production of substantially pore-free and defect-free components.
Further in accordance with the invention a component produced using the abovementioned process is claimed: